Bacillus anthracis Targets Involved in Chemokine-Mediated Antimicrobial Activity Chemokines are chemotactic cytokines that function in host defense by orchestrating leukocyte migration to sites of infection. However, a number of chemokines have also been found to directly kill a range of pathogenic microorganisms through an as yet undefined mechanism. We previously reported that the interferon-inducible CXC chemokines, CXCL9, CXCL10, and CXCL11, block Bacillus anthracis spore germination, reduce spore viability, and kill vegetative cells, with CXCL10 being the most effective. We also reported that C57BL/6 mice, which are resistant to pulmonary B. anthracis Sterne strain infection, produced significant levels of CXCL9, CXCL10, and CXCL11 in their lungs following spore challenge; whereas, highly susceptible A/J mice did not generate significant levels of these chemokines during pulmonary infection. In vivo neutralization of CXCL9, CXCL9/CXCL10, or CXCL9/CXCL10/CXCL11 rendered C57BL/6 mice susceptible to pulmonary anthrax whereas neutralization of their shared receptor CXCR3, which is the receptor expressed on leukocytes recruited to the site of infection by CXCL9, CXCL10, CXCL11, had no impact on survival. These findings support that CXCL9, CXCL10, and CXCL11 have direct antimicrobial effects against B. anthracis both in vitro and in vivo. To identify the vegetative cell target(s) of CXCL10, we screened a B. anthracis transposon mutant library and found that disruption of ftsX, which encodes the transmembrane protein of a widely conserved prokaryotic ABC transporter, resulted in a CXCL10-resistant phenotype. Deletion of the ftsX gene in B. anthracis (i.e., DftsX) resulted in resistance of vegetative cells to CXCL10, and complementation of ftsX restored CXCL10 susceptibility. In contrast, DftsX spores remained susceptible to CXCL10, suggesting that spores have a different CXCL10 target. To further investigate the role of FtsX, as well as other B. anthracis targets of CXCL10, we propose three Specific Aims: 1) Determine the role of FtsX in susceptibility of vegetative cells to CXCL10; 2) Identify spore target(s) of CXCL10; and 3) Determine the role of spore and vegetative bacterial targets of CXCL10 during in vivo infection. These studies will provide a key foundation for the development of innovative therapeutic strategies for treating infections caused by not only B. anthracis but also a range of pathogenic, potentially multi-drug resistant microorganisms.